1. Field of the Invention
This invention relates to induction motors generally and, more particularly, to induction motors capable of developing nominal operating speeds in excess of 3600 rpm from 60-cycle a-c power.
2. Description of the Related Art
Conventional two-pole induction motors powered by 60-cycle a-c (alternating current) have a maximum nominal speed of 3600 rpm. These motors are driven by a rotating electromagnetic field, the velocity of which is a function of the frequency of applied power and the number of poles for which the stator is wound. For a two-pole motor, the rotating field makes one complete revolution in one cycle of the current. For a motor that is wound for p poles, the rotating field moves past one pair of poles in one cycle. With a frequency of f cycles per second for the applied power, the rotating field travels at f cycles per second divided by p/2 cycles per revolution equals 2f/p revolutions per second. The speed of rotation of the rotating field is the synchronous speed n.sub.s of the motor. Since this is usually expressed in revolutions per minute, the formula for the synchronous speed is as follows: EQU n.sub.s =60 (sec./min.).times.2f/p=120 f/p (1)
The highest synchronous speed obtainable is that for a two-pole motor. Putting the numbers for a 2-pole, 60-cycle motor in Equation (1) yields: EQU n.sub.s =120.times.60/2=3600 rpm (2)
For a four-pole, 60-cycle motor, the synchronous speed is 1800 rpm; for a six-pole, 60-cycle motor, the synchronous speed is 1200 rpm, etc. Whereas synchronous motors generally operate at synchronous speed, induction motors usually develop a slippage under load so that a four-pole induction motor having a nominal speed of 1800 rpm commonly operates at about 1760 to 1780 rpm.
Insofar as is known, attempts to develop rotational shaft speed in excess of synchronous speed for alternating current induction motors have not been successful. A motor with multiple rotors is the subject of U.S. Pat. No. 2,864,017 to Waltscheff. The Waltscheff apparatus uses one conventional rotor, one or more intermediate rotors, torque-increasing gears and one or more clutches. The movement of the additional rotor or rotors of the Waltscheff apparatus serves to transmit the rotation of the additional rotor or rotors through over-running clutches and gears to the main rotor in order to increase the main rotor's torque. However, the speed of the output shaft is no greater than that of a conventional single-rotor machine.
Arrangements in accordance with the present invention are capable of developing output shaft speeds in excess of 3600 rpm from 60-cycle a-c power through the use of multiple rotors, one of which is coupled to drive what becomes a rotating "stator" (rotating field structure) for a second rotor which drives the output shaft. U.S. Pat. No. 3,683,217 of Agalakov et al discloses an electric motor which has a plurality of rotor cores. However, it appears that these are fastened together so that they rotate as a single rotor and do not increase shaft speed.